1. Field of the Invention
The present invention relates to a semiconductor substrate cleaning and more particularly, to a cleaning method and a cleaning system of a semiconductor substrate, and a production method of a cleaning solution used for the method, which are used for semiconductor device fabrication.
2. Description of the Prior Art
Recently, with the decreasing size and increasing integration-level of semiconductor devices, it has become indispensable during semiconductor device fabrication to clean the surface of a semiconductor substrate between fabrication process steps, thereby removing various contaminants adhered onto the surface.
To perform the cleaning process as above, wet cleaning treatment with the use of chemicals such as high-cleanliness organic solvents, strong acids, and strong alkalis has been employed. In this treatment, it is inevitable to use a large quantity of these chemicals at various stages.
For example, in the cleaning process for removing metallic contaminants on the surface of a semiconductor substrate, a large quantity of such a chemical solution as that prepared by mixing hydrochloric acid (HCl) (37% by weight) and hydrogen peroxide (H.sub.2 O.sub.2) (30% by weight) with pure water at a volume ratio of 1:1:6 (which is called as an "hydrochloric acid hydroperoxide mixture (HPM) is used.
Use of such high-purity and high-concentration chemicals significantly affects the earth environment. Also, the cost of producing these chemicals and of disposing wastes generated therefrom is very high. Therefore, the industry has begun to review wet cleaning treatment which consumes a large quantity of the chemicals.
The Japanese Non-Examined Patent Publication No. 56-150818 published in 1981 discloses, although the application field is different from that of the invention, a cleaning method for removal of bacteria growing in the pure-water supplying piping system without using high-concentration chemicals. In this method, hypochlorous acid (HClO) and hypochlorous acid ions (HClO.sup.-) are used as a cleaning agent, which are obtained by dissolving a trace of chlorine (Cl.sub.2) gas in pure water to react therewith by bubbling of the chlorine gas. In this case, the cleaning agent containing the hypochlorous acid and hypochlorous acid ions are used as an alternative to simple pure water.
In this conventional method, if the cleaning agent contains a large quantity of chlorine gas, the chlorine tends to be left on the surface of the semiconductor substrate after the cleaning process. The remaining chlorine may corrode the aluminum (Al) wiring or interconnection of the semiconductor devices, resulting in the possibility of property or performance degradation of the semiconductor devices. In consideration of this effect, the concentration or content of the contained hypochlorous acid and hypochlorous acid ions is held to 0.1 ppm or below, respectively.
The inventor, Y, Shiramizu, and other researchers created and filed on Mar. 25, 1994 an invention relating to an epoch-making wet treatment method as the Japanese Patent Application No. 6-56106, which was disclosed in the Japanese Non-Examined Patent Publication No. 7-263391 published in October 1995. This method allows great decrease of the necessary amounts of high-purity and high-concentration chemicals.
With this method, pure water containing an extremely small amount of an electrolyte is prepared and then, it is electrolyzed to produce a special water, i.e., "electrolysis-activated water". The special water thus obtained is used for a wet treatment of the semiconductor substrate.
For removal of metallic impurities on the surface of the semiconductor substrate, a halogen or halide gas is used as the above electrolyte. In this case, the "electrolysis-activated water" is generated by the following process steps. First, a gaseous mixture containing at least one of chlorine (Cl.sub.2) gas, hydrogen chloride (HCl) gas, hydrogen bromide (HBr) gas, and hydrogen iodide (HI) gas is prepared. Next, this mixture is dissolved into pure water in an electrolysis chamber or tank. An electrolysis reaction of the water is then caused by supplying electric energy, thereby generating the "electrolysis-activated water" from the anode area of the tank.
The metallic impurities left on the semiconductor substrate for some cause are typically bonded to silicon (Si) or exist as oxides and therefore, they are difficult to remove. Organic impurities occurring from remaining photoresists or the like are difficult to completely remove even with an oxidizing cleaning agent.
With the conventional cleaning method disclosed in the Japanese Non-Examined Patent Publication No. 56-150818, chlorine-containing pure water in which the contents of hypochlorous acid and hypochlorous acid ions are held to 0.1 ppm or below is used as a cleaning agent instead of simple pure water. However, the chlorine contained in the cleaning agent exists as "free chlorine" and consequently, it is not stable. For example, it can readily generate chlorine ions under heat or the like. Thus, this conventional cleaning agent is not practically effective if it is used for removing metallic impurities.
The conventional wet treatment method filed as the Japanese Patent Application No. 6-56106 has an object of decreasing greatly the necessary amounts of high-purity chemicals, and this object can be accomplished with the use of the "electrolysis-activated water". However, in this method, to increase the efficiency of electrolysis, a high voltage needs to be applied to the "electrolysis-activated water" together with the addition of an electrolyte. Thus, it is difficult for this method to significantly reduce power consumption.
Further, the "electrolysis-activated water" has a disadvantage of instability at room temperature. The instability leads to high cleaning performance or effect on one hand, however, it means that the cleaning effect may be lowered before the time of a cleaning process on the other hand. Therefore, the instability is a critical defect for some cleaning processes in which the stability of the cleaning effect is essential.
For example, with the conventional wet treatment method filed as the Japanese Patent Application No. 6-56106, the cleaning effect lowering or degradation of the anode water obtained in the anode area of the electrolysis tank is less than that of the cathode water which is obtained in the cathode area thereof. Even for the anode water whose cleaning effect lowering is relatively less, the Oxidation Reduction Potential (ORP) at room temperature is lowered from 1200 mV to below 1000 mV just after the finish of production in approximately 70 hours. On the other hand, the hydrogen ion concentration, i.e., pH, is kept unchanged.
For removal of the metallic impurities adhered on semiconductor substrates, not only the pH value of the cleaning agent but also the ORP value thereof govern the removing or cleaning performance. For removal of the organic impurities adhered on semiconductor substrates, only the ORP value governs the removing performance. Thus, the above ORP lowering of the cleaning agent is critical for these impurities.
With the conventional wet treatment method filed as the Japanese Patent Application No. 6-56106, it is very difficult to prevent the cleaning effect of the "electrolysis-activated water" from lowering. In other words, there are limitations in maintaining the stability the cleaning property of the "electrolysis-activated water" as with popular chemicals in the above conventional cleaning methods.